1. Field of the Invention
This invention relates to rubber articles intended to be capable of resisting high pressure and cyclic operation under such high pressure.
2. Description of the Prior Art
The testing of pipelines and in particular the joints thereof is conventionally effected by applying a high pressure to a section of a pipe and then monitoring any drop in the applied pressure which would indicate leakage. It has also been proposed to limit the length of pipe tested at any given time by inserting an obturating tool into the pipeline and then activating at least two seals to isolate the given section and once again to monitor the pressure drop, if any. Such seals are an essential feature of any obturating tool and many such tools have been proposed in the patent literature. However, most such tools have not been employed practically since a simple block of rubber which is either inflated to cause sealing contact with the pipe wall or is subjected to a compression force so that it expands by the action of the poisson effect is liable to be unsatisfactory in practice inter alia for the following reasons.
The very high pressures which are inevitably employed tend to cause the seals to perform as a vey viscous fluid and an extrusion effect is produced by these high pressures.
It is also desirable with obturating tools of the kind in question that they should be capable of being repositioned at successive locations along a pipe line and to do this it is necessary to release the pressure in order that the obturating tool can be moved. Ideally it is possible to complete each test over a time period of approximately one minute and this substantial benefit will be lost if the sealing rubbers fail to react rapidly both to the reduction in pressure and to the increase in pressure just before the seals are to be activated. It follows that the basic rubber must have a rapid response to changes in pressure and must have a poor "memory" so that the rubber does not readily set in a given configuration.
It has already been proposed to construct a piston ring of rubber with layers of absorbent fabric adhered to and covering parts of the external surface in U.S. Pat. No. 3,583,712, but as incorporated in a groove of a piston the piston ring is not subjected to multiple compression cycles involving pressures of the order of 6000-12000 lb/sq. in. (400-800 kg/sq.cm.).
Extrusion of the material of the ring cannot take place because the end surfaces are confined by rigid rings of the L-section. Used alone, the proposed piston ring could not withstand the extreme pressures and cyclic conditions encountered by seals of the obturating tools under consideration, that is as disclosed in co-pending British Patent Application 80.25902 (Publication No. 2,056,091) and other tools involving very high pressure conditions for the seals.